Blind rivets are special rivets that can be set without access to the rear side of the workpiece. Blind rivets find special utility in many applications, and especially in aircraft construction. These rivets are available in many designs, but there are three general types, namely, screw, mandrel and explosive. In the mandrel type, the rivet takes the form of a sleeve, and the rivet is set as the mandrel or stem is pulled through the sleeve. The present invention is concerned with the mandrel type of blind rivet.
Problems have been encountered in the past in the mandrel type of blind rivet, in that when the mandrel is provided with a sufficient diameter to upset the rear end of the sleeve to cause it to clamp against the rear side of the workpiece, the excessive diameter of the mandrel as it is drawn into the sleeve tends to cause dimpling and distortions of the workpiece, especially when the workpiece is relatively thin.
A three-part blind rivet is provided, consisting of stem, sleeve and collar. The stem comprises a means for causing the collar to effectuate the multiple functions of flaring outwardly the rear of the sleeve; and locking together the stem and the sleeve by being swaged into radially aligned annular recesses, namely a flare at the forward end of the sleeve, and a locking groove in the stem. The collar also contributes to radial outward expansion of the sleeve, thereby causing the rivet to fill the bore or hole in the workpieces in which it is secured. The collar is tightly swaged into the locking groove as it is moved by the stem into the sleeve, and then into the sleeve flare by being moved against an external anvil.
The blind rivet of the present invention overcomes the disadvantages of the prior art mandrel-type blind rivet, by providing the mandrel with an outer diameter only slightly greater than the inner diameter of the sleeve. Then, when the mandrel is drawn into the sleeve, its diameter is capable of creating an outward radial clamping action of the sleeve against the workpiece, which is desired, but which is not sufficiently great to cause dimpling or distortions in the workpiece, even though the workpiece may be relatively thin.
The desired upsetting of the rear end of the sleeve against the workpiece is achieved by the rivet of the present invention by means of a separate collar which is composed of a malleable material, and which is slidable on the forward section of the stem or mandrel. As the stem is drawn into the sleeve during the setting of the rivet, the collar slides back along the forward section and surrounds a locking groove located along the stem, separating the forward section of the stem, from the rear section which has an enlarged head of slightly greater diameter. As the stem is drawn into the sleeve, the locking collar engages an annular or radial shoulder defined by the head at the rear edge of the groove. The collar bears against and flares or upsets outwardly the rear end of the sleeve and clamps it against the workpiece. In the process, the locking collar is drawn or swaged down into the locking groove, and the locking collar is then moved or drawn through the sleeve, together with the rear section of the stem as the setting continues. The locking collar and rear section of the stem have sufficient diameter to produce a desired radial outward action against the bore of the sleeve, to cause the sleeve to be clamped against the bore of the hole in the workpiece without creating excessive stresses which would cause distortions in the workpiece.